1. Field of the Invention
The present invention relates to spectacles for presbyopia by means of which a user can see objects not only in near range, but also in a middle to far range, and more particularly relates to spectacles for presbyopia, with which a person having both presbyopia and astigmatism against rule can clearly see objects in a middle to far range during a time in which the person looks at objects in near range through presbyopia correcting lenses.
2. Related Art Statement
First of all, the principal matters of ophthalmology will be explained. An adjusting power of eyes, i.e. eyesight for seeing objects at a visualization distance of 25-30 cm is generally 3-4 D (diopters), but this eyesight is gradually decreased in accordance with aging and aged persons could not see objects in a near range clearly. Such a situation of eyes is called presbyopia. In general, the presbyopia appears after forty five years old at which eyesight has decreased to 2-4 D.
A nearsighted person can see objects in a near range rather clearly, and therefore although his or her adjusting power of eyes is decreased due to aging, he or she can see objects in a near range rather clearly by reducing a degree of shortsightedness of spectacles or putting off the spectacles or using week convex lens or lenses.
However, when eyesight of a normal-sighted or far-sighted person is decreased due to aging, the adjusting power of eyes for seeing objects in a near range must be corrected by spectacles including convex eyeglass lenses. Such spectacles are called spectacles for presbyopia or far-sighted glasses.
Astigmatism is such a refracting condition of an eye that a light ray emitted from a point outside the eye is not focussed at a point within the eye or even outside the eye. In a regular astigmatism, a vertical meridian and a horizontal meridian cross each other at right angles, and a focal line of the vertical meridian and a focal line of the horizontal meridian are shifted back and for along an optical axis. The regular astigmatism may be classified into astigmatism with the rule in which the strongest main meridian is vertical and astigmatism against the rule in which the strongest meridian is horizontal. Strictly speaking, almost all eyes have the astigmatism to a certain extent.
As stated above, eyesight is gradually decreased in accordance with aging and a degree of presbyopia increases, and at the same time, the astigmatism against the rule is liable to increase in accordance with aging. This is due to the fact that a curvature of a vertical meridian of a cornia becomes smaller to reduce the astigmatism with the rule, and the astigmatism of a crystal lens which has a tendency of the astigmatism against the rule is increased (Ophthalmologic Book, No. 29, “Old man and Eyes”, page 7). According to “Refracting Abnormality and Correction for the Same (Revised Edition 3)”, published by Kinbara Publisher, page 136, 66.9% of persons of sixty to eighty years old have the astigmatism against the rule.
FIG. 15 is a schematic view showing the astigmatism against the rule. The astigmatism against the rule is a kind of the regular astigmatism, and thus the horizontal meridian crosses the vertical meridian at right angles. In the astigmatism against the rule, the horizontal meridian has a strong refraction power (strong main meridian), and a light ray emitting from a point at infinity is focused at a front focal line and the vertical meridian has a weak refracting power (weak main meridian) and a light ray from a point at infinity is focused at a back focal line.
When eyesight is decreased due to aging, eyes generally become far-sighted, and in the astigmatism against the rule, the front focal line is much closer to a retina than the back focal line. A range between the front focal line and the back focal line is called a focal range or focal depth which represents a degree of the astigmatism. A portion of the focal range slightly closer to the front focal line is called a circle of least confusion.
Usually, a person with the astigmatism sees objects such that eyes are focused at the circle of least confusion. If the person has a sufficiently strong focus adjusting power, either one of the front focal line and back focal line which causes less fatigue to eyes is focused on a retina, and an object can be seen clearly.
In case of the astigmatism against the rule of aged or old persons, the presbyopia becomes dominant and a degree of the astigmatism against the rule is increased in accordance with aging. Then, the focal range becomes larger. When objects are seen with naked eyes, the circle of least confusion on which object images are focused is far away from retina. Since eyesight is decreased, it is no more possible to focus a front focal line on retina, and thus vague images of objects in a middle to far range are formed on retina.
Aged persons usually have both the presbyopia and the astigmatism against the rule, and there have been developed spectacles having combination lens of convex lenses for correcting near-sightedness and cylindrical lenses for correcting the astigmatism against the rule. Such spectacles are very useful for seeing objects in a near range as well as objects in a middle to far range. However, upon assembling and accommodating such spectacles, a precise measurement of the astigmatism of a user has to be carried out several times, and therefore such spectacles are generally very expensive. Furthermore, it is rather difficult to attain correct accommodation, and sometimes problems of over-correction and distortion might occur.
When an aged person wearing spectacles for presbyopia is looking objects in a near range, he or she often wants to see objects in a middle to far range over his or her far-sighted glasses. Since almost all aged persons have the astigmatism against the rule, when they see objects in a middle to far range with naked eyes, it is difficult to see clear images of the objects. However, since such a situation could not affect the daily life seriously, aged persons do not feel inconvenience so much.
However, aged persons sometimes wish to look at objects in a middle and far range over spectacles for presbyopia much more clearly. For instance, aged persons often want to see detail appearance of faces of grandchildren just coming home, relatively small characters in TV screens, and birds and flowers outside windows.